In rats, muscle glycogen depletion has been associated with increased insulin action. Whether this also occurs in man has not been reported. After four days of rest, thirteen males (E group) had a percutaneous muscle biopsy of the vastus lateralis muscle before and after a euglycemic clamp at plasma insulin concentrations of approximately 100 and 1900 MuU/ml, with simultaneous indirect calorimetry. This was repeated one week later, but after glucogen depleting exercise the night before the euglycemic clamp. Seven subjects underwent the same protocol but were also refed 100 grams of carbohydrate after the exercise (EF group). In both groups the mean muscle glycogen content was approximately 40% lower after exercise compared with the muscle glycogen content measured after rest. In the E group, the mean muscle glycogen synthase activity increased three-fold after exercise, but increased only two-fold in the EF group. In both groups the basal and insulin stimulated glucose oxidation rates were lower in the post exercise condition. The insulin stimulated glucose storage rate increased significantly in the E group after exercise but not in the EF group. In the E group, total insulin mediated glucose disposal (M) was 17% and 10% higher after excercise during the low and high dose insulin infusion, respectively. No significant changes in M were observed in the EF group. For all subjects M correlated with the carbohydrae storage rates during the low and high dose insulin infusion. In the E group the mean muscle glycogen synthase activity increased three-fold after exercise but increased only two-fold in the EF group. We concluded that muscle glycogen depletion is associated with increased in vivo insulin action in man. The increased insulin action is correlated with increased muscle glycogen synthase activity.